This invention relates to apparatus and methods of gas adsorption.
Separations of gases have long been important in gas purification processes such as used industrially in gas purification. Removal of carbon dioxide continues to be an important objective for purifying air for humans to live underwater and in space. Other important technologies that can utilize improvements for gas separation include: fuel cells, ammonia production, fertilizer manufacture, oil refining, synthetic fuels production, natural gas sweetening, oil recovery and steel welding.
The adsorption capacity of a gaseous species onto an adsorbent is commonly expressed in graphical form in adsorption isotherms and isobars, which are widely published in the literature and by adsorbent manufacturers and suppliers. For the sorption of gas species, the capacity is typically expressed as the equilibrium mass of the species sorbed per unit mass of adsorbent (e.g., kg species/100-kg adsorbent). The sorbent capacity varies as a function of temperature and the partial pressure (concentration) of the species being sorbed. Loading or capacity typically increases as the adsorbent bed temperature decreases or the partial pressure of the sorbed species in the gas phase increases.
The variation of adsorption capacity with temperature and pressure can be used to effect separations of gas species. For example, in pressure swing adsorption (PSA) gas species are adsorbed onto a sorbent at relatively high pressure, tending to remove the species from the feed stream. In a regenerative PSA process, reducing the absolute pressure (e.g., applying a vacuum) to the loaded sorbent bed or reducing the partial pressure of the sorbed species in the gas phase by sweeping a lower concentration purge gas through the bed regenerates the sorbent. Cycle times for PSA processes are typically measured in minutes (Humphrey and Keller, xe2x80x9cSeparation Process Technology, McGraw-Hill, 1997). In a regenerative temperature swing or thermal swing (TSA) absorption process, species are adsorbed at low temperature where the loading capacity is relatively high and (at least partially) desorbed at higher temperature, thus recovering sorption capacity for additional cycles.
In addition to gas species separations, TSA can be used to thermochemically compress gases. Sorption based thermochemical compression is applicable to refrigeration and heat pump cycles (e.g., see Sywulka, U.S. Pat. No. 5,419,156) and for chemical processing in general.
Gas adsorption is known to be applicable to a wide range of gas species (see, e.g., Kohl and Nielsen, Gas Purification, 5th Ed., Gulf Publ. Co., Houston, Tex.). Kohl and Nielsen report that in conventional TSA gas purification processes, adsorbent bed loading and unloading cycles are typically on the order of hours.
Despite their long-known use and importance, multiple problems remain with gas adsorption separation technologies. These problems include: use of excess energy, bulky apparatus or low capacity, cost, and slow rate and/or low mass of gas separated.
In one aspect, the present invention provides a sorption pump that includes an adsorption layer comprising an adsorption mesochannel containing adsorption media, and a heat exchanger in thermal contact with the adsorption layer. The heat exchanger includes at least one microchannel. The adsorption layer has a gas inlet such that gas directly contacts the adsorption media without first passing through a contactor material.
In another aspect, the invention provides gas adsorption and desorption apparatus that includes at least one adsorption layer comprising an adsorption mesochannel containing adsorption media. The adsorption mesochannel has dimensions of length, width and height; wherein the height is at least 1.2 mm. The apparatus possesses capability such that, if the adsorption media is replaced with an equal volume of 13xc3x97zeolite, having a bulk density of about 0.67 grams per cubic centimeter, and then saturated with carbon dioxide at 760 mm Hg and 5xc2x0 C. and then heated to no more than 90xc2x0 C., at 760 mm Hg, then at least 0.015 g CO2 per mL of apparatus is desorbed within 1 minute of the onset of heating. By heated to xe2x80x9cno more than 90xc2x0 C.xe2x80x9d typically means that 90xc2x0 C. water is passed through the heat exchanger; however, the phrase also encompasses heating by other means such as an electrically-resistive heater. Preferably, the apparatus includes at least one heat exchanger in thermal contact with the adsorption layer.
In yet another aspect, the apparatus is configured to selectively heat the adsorbent. The at least one heat exchanger could be configured such that the heat exchange fluid flow paths substantially overlap the area of adsorption channel or channels. Alternatively, the apparatus could contain a relatively thermally conductive material overlapping the adsorption channel or adsorption channels and a relatively thermally insulating material that does not substantially overlap the adsorbent channel or adsorbent material. By xe2x80x9csubstantially overlapxe2x80x9d it is meant that, when viewed from a direction perpendicular to the direction of flow in which the adsorption channel and heat exchanger is stacked, the areas of the adsorbent channel(s) and the thermally conductive material have at least about an 80% overlap.
In a further aspect, the invention provides a sorption pump, that includes an adsorption layer comprising an adsorption channel containing adsorption media, and a mesochannel heat exchanger in thermal contact with the adsorption layer. The mesochannel heat exchanger has a fluid flowing therethrough that has a high thermal diffusivity, such that the characteristic heat transport time for the fluid in combination with the mesochannel heat exchanger is no greater than 10 seconds.
The invention also provides an apparatus in which adsorption/desorption cells are connected to improve overall energy efficiency. Each cell contains at least one adsorption mesochannel having an inlet and/or outlet. Typically, each cell contains multiple adsorption mesochannels that share a common header and common footer, and that are operated together. Preferably, each adsorption channel is in thermal contact with at least one heat exchanger. Each adsorption channel contains adsorption media. Typically, the apparatus also contains or is used in conjunction with a heat source and a heat sink. In some embodiments, the heat sink could be the non-adsorbed gas, which is passed through and removed from the apparatus. The apparatus contains heat transfer conduits between each cell and the heat source and heat sink and also contains heat transfer conduits between each cell and at least two other cells. In operation, the conduits carry a heat exchange fluid or can contain a thermally conductive material. The apparatus also contains valves that can control gas flow into the at least one adsorption channel. Cell volume is defined as the volume of the adsorption channel or channels that are operated together, including the volume of the heat exchange channel or channels, the volume between such channels, the volume of the outer walls of the cells, and the volume of inlet and outlet footers, when present.
The invention further provides a method of gas adsorption and desorption, comprising passing a gas into an adsorption layer where at least a portion of the gas is adsorbed onto adsorption media to form an adsorbed gas and selectively removing heat from the adsorption layer through a distance of 1 cm, preferably 2 mm, or less into a heat exchanger; and, subsequently, selectively heating the adsorption media through a distance of 1 cm, preferably 2 mm, or less from a heat exchanger, and desorbing gas. The gas directly contacts the adsorption media without first passing through a contactor material. For more rapid heat transfer (and thus faster cycling), the adsorption channel may contain heat transfer agents such as metal fins or pins, or graphite fibers.
The invention also provides a method of gas adsorption and desorption, comprising passing a gas into an adsorption layer where at least a portion of the gas is adsorbed onto adsorption media to form an adsorbed gas and selectively removing heat from the adsorption layer through a distance of 1 cm or less into a heat exchanger; and, subsequently, selectively heating the adsorption media through a distance of 1 cm or less from a heat exchanger, and desorbing gas.
The invention also provides a multi-cell sorption pump, comprising: at least six sorption cells; where each sorption cell comprises at least one adsorption layer, and at least one heat exchanger layer. Thermal connections connect each sorption cell to at least two other sorption cells and to a heat source and to a heat sink, such that each sorption cell can cycle thermally from adsorption to desorption and back to adsorption by sequentially receiving heat from said at least two other sorption cells prior to receiving heat from the heat source, and then sequentially giving up heat to at least two other sorption cells prior to giving up heat to the heat sink, such that thermal recuperation is provided.
The invention also provides a method of gas adsorption and desorption, comprising a first step of passing a gas into a first adsorption layer containing a first adsorption media where at least a portion of the gas is adsorbed onto the adsorption media to form an adsorbed gas and removing heat from the adsorption layer through a distance of 1 cm or less into a first heat exchanger. Subsequently, in a second step, the adsorption media is heated through a distance of 1 cm or less from the first heat exchanger, and gas is desorbed. Simultaneous with the first step, a heat exchange fluid flows through the heat exchanger and exchanges heat with the adsorbent. This heat exchange fluid flows into a second heat exchanger that, in turn, exchanges heat with a second adsorption layer containing a second adsorption media.
The invention also provides a method of gas adsorption and desorption that includes: a first step of transferring heat from a heat source into at least two first cells and desorbing gas from each of the two first cells, and transferring heat from at least two second cells to at least two third cells; a second step of transferring heat from the at least two second cells to a heat sink, and adsorbing gas into the at least two second cells, transferring heat from the at least two first cells to the at least two third cells; a third step of transferring heat from a heat source into the at least two third cells, and desorbing gas from each of the at least two third cells, transferring heat from the at least two first cells to the at least two second cells; and a fourth step of transferring heat from the at least two first cells to a heat sink, and adsorbing gas into the at least two first cells, transferring heat from the at least two third cells to the at least two second cells. In this method, each cell comprises at least one sorbent, and at least one heat exchanger.
The invention also provides a method of adsorption and desorption that provides the thermal enhancement of PSA adsorption, thereby obtaining greater utilization of the adsorbent media than would be accomplished by PSA adsorption alone. This includes cooling of the adsorbent media during adsorption at one partial pressure of the adsorbing specie(s), so that a greater amount of adsorbing specie(s) can be adsorbed, and/or heating of the adsorbent media during desorption at a lower partial pressure of the desorbing specie(s), so that a greater amount of desorbing specie(s) can be desorbed. In general, the methods described herein are applicable for thermal swing adsorption, thermally-enhanced pressure swing adsorption, and thermochemical compression.
In a report (xe2x80x9cMicroscale Adsorption for Energy and Chemical Systemsxe2x80x9d) appearing on the PNNL web site in May 2000, Viswanathan, Wegeng and Drost reported the results of calculations and experiments for investigations of microchannel adsorption with short cycle times. From the reported estimate that 95% of CO2 reaches the zeolite particles in 30 seconds, based on semi-infinite diffusion, it is clear that this calculation involves zeolite adsorbent in a xe2x80x9cflow-byxe2x80x9d arrangement, rather than a xe2x80x9cflow-throughxe2x80x9d arrangement. A xe2x80x9cflow-byxe2x80x9d arrangement is one in which adsorbent occupies less than the full cross-sectional area of the flow path and gas flow is primarily by an adsorbent, requiring that contact of the adsorbent media, with the specie(s) to be adsorbed, occur primarily by mass diffusion into and through the adsorbent structure, while in a xe2x80x9cflow-throughxe2x80x9d arrangement the adsorbent is substantially placed within the flow path, so that the fluid flows directly xe2x80x9cthroughxe2x80x9d rather than adjacent to the adsorbent structure.
Various embodiments of the invention can provide numerous advantages including one or more of the following: rapid cycling, rapid sorbent regeneration, reduced time and/or larger volumes of gas adsorbed as a function of sorbent mass required, excellent device stability, low cost, direct sorption into the sorption media without requiring diffusion through a contactor, preferential heating/cooling of the sorption media to a greater degree than other elements of the adsorber structure, configurations of sorption units with recuperative heat exchange thereby allowing energetically efficient temperature swing separations and/or more energetically-efficient, thermochemical compression.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.
In the present invention, the term xe2x80x9cmicrochannelxe2x80x9d refers to a channel with at least one dimension, of 1 mm or less, preferably in a direction perpendicular to net flow through the channel. The term xe2x80x9cmesochannelxe2x80x9d refers to a channel with at least one dimension, in a direction perpendicular to net flow through the channel, of 1 cm or less.
The xe2x80x9ctheoretical capacityxe2x80x9d of an amount of adsorbent is determined by maintaining the adsorbent at a first temperature, at a fixed partial pressure for the gas specie(s) to be adsorbed, for a sufficient period of time so that essentially no more gas will be adsorbed, then shutting off the gas flow and heating to a second temperature to desorb gas, at the same or another fixed partial pressure for the gas specie(s), until essentially no more gas is desorbed, and measuring the amount of gas desorbed; the amount of gas desorbed is defined to be the xe2x80x9ctheoretical capacityxe2x80x9d of an adsorbent material for that set of process conditions. The actual xe2x80x9ccapacity utilizedxe2x80x9d within a working sorption pump is measured at the same pressure and temperature conditions, but for a selected, finite period of time, and therefore may be less than the theoretical capacity.